Melamine, the industrial chemical at the centre of the tainted milk scandal, may have been routinely added to China's fish to artificially boost protein content.

New research suggests that edible fish in fish that have been fed melamine contains residues of the nitrogen-rich substance. Fish feed from China have been found contaminated with melamine before.

China is the world's largest seafood producer, exporting billions of dollars worth of seafood. The US alone imported about US$2 billion of seafood products from China in 2007, according to the USDA.

The US Food and Drug Administration (FDA) do not require seafood products to be screened for melamine, though some US fish importers have taken to voluntarily testing. However, laboratory studies by the FDA scientists found that fish tissues had melamine concentrations of up to 200 parts per million, which is 80 times the maximum tolerable amount set by the FDA for safe consumption.

In the US, fish from China can be found in the frozen food section in supermarkets and is served in posh restaurants. Boston-based Stavis Seafoods has been conducting its own testing but has not found any melamine in the catfish and tilapia imported from China.

Some scientists said testing of melamine in farm-raised fish from China should be made mandatory due to the lack of information about melamine levels in Chinese feed and fish.

Jim Riviere, director of chemical toxicology research at North Carolina State University said it would be wise to screen for melamine as no one knows how much concentration and for how long fish from China have ingested melamine.

A research conducted by Riviere shows that unlike fish, melamine in feed consumed by pigs and cows is excreted, leaving no trace of it in the muscle or meat of the animals.

Since last year, the FDA has restricted entry of seafood from China unless the shipments come with an independent lab report certifying the seafood is free of harmful additives such as carcinogen malachite green, which some Chinese fish farms use to control disease. Melamine, however, is not included on the list of additives.

Shipments of unsafe seafood from China continue to be detained at US ports, exposing holes in a food safety system that is riddled with problems.

Last month, 26 containers of seafood from China were refused entry in US ports. Inspectors cited a variety of reasons including salmonella, unsafe additives, unapproved drugs and labelling problems, said the FDA.

Karunasagar, the UN's fishery expert, said China needs to tackle the problem at the source and monitor melamine in feed.

That is not an easy task though, as US attorney Steve Dickinson said there may be hundreds of thousands of feed sources in China compared to just a handful of approved suppliers in the US.

Fang Shijun, manager of feed research at Shanghai eFeedLink Information Technology, an agricultural consulting and research firm, said he believes the adulterated products are being supplied only by small operations which abound in China and that it is impossible to calculate how many of them have done that.

Fang said there may not be many feed suppliers adding melamine today, due to public awareness and the government's publicised crackdown, but the problem has not been eradicated.

Fang said more government supervision and more random checks are needed, and regulations and standards must be fully executed.

Melamine, an industrial chemical used to make plastics, can lead to kidney problems.

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